Late last week I did something I've done a couple of times before with other works-in-progress, although only reluctantly and after great deliberation. I stopped only partway through the first draft, backed up, and started over. No, I didn't throw out everything; I'm not quite that crazy.
My general process when I work on a new book goes something like this:
Development of the idea and characters.
General 30,000-foot outline.
Write first draft from the outline, and collect notes towards revisions to apply to the next draft.
Create new outline from the first draft, because the first draft often evolves drastically away from the outline.
Repeat steps 3 and 4 with each draft, applying collected revisions each time.
Sometimes, in the middle of step 3, I realize there are major changes that need to be made to some part of the story I've already written. New perspectives that can't be found in the outline phase can arrive in the middle of writing a draft, and that perspective may alter the course of the story. In cases like that, it's best to back up, rewrite the parts of the story already extant to implement those changes, and then continue.
I was about 30,000 words into the first draft when I realized I'd passed up the chance to make some significant improvements in the story that would pay off major dividends later on down the line. Not the kind of thing where I could fake it and pretend I'd already written the earlier material in that vein, because its very presence would significantly alter the course of the goings-on. Best to back up, make the changes, and proceed from there.
One thing I try to be conscious of during the whole process is leaving opportunities on the table. If there's a small change I can make to a story that significantly expands how it explores its world and makes story possibilities out of its ingredients, I try to alert myself to those possibilities and leverage them.
It always depresses me when I encounter a finished work by someone else that seems like it's leaving possibilities on the table. I know no work can ever be perfect, and every work is always both guided and constrained by the sensibilities and constitution of its creator. The essence of art is selectivity. Sometimes the way a work would seem to be leaving opportunity on the table is just a way for it to have constraints that shape it according to a higher vision.
But sometimes it's just a missed chance to do something truly interesting. I can't be conscious of every single thing I might leave out, but when they do come to mind I owe it to myself to make sure they don't slide all the way through my fingers.